Crude oil production from oil in subterranean reservoirs may involve use of various flooding methods as the natural forces that are used in the “primary recovery” process become depleted. A large portion of the crude oil may have to be driven out of the formation in “secondary” or “tertiary” recovery processes. In addition, some reservoirs may not have sufficient natural forces for oil production even by primary recovery processes. The production of crude oil using such flooding methods is one example of an enhanced oil recovery process.
One trend in the petroleum industry now is to plan exploitation of the oil field at the beginning of the project. Injecting a displacing fluid or gas may begin early, long before the complete depletion of the field by primary recovery processes. Methods for improving displacement efficiency or sweep efficiency may be used at the very beginning of the first injection of a displacing fluid or gas, rather than under secondary and tertiary recovery conditions.
The least expensive and easiest method of flooding a subterranean reservoir for the production of crude oil is by injecting a liquid or a gas into the well to force the oil to the surface. Water flooding is the most widely used fluid. However, water does not readily displace oil because of the immiscibility of water and oil and/or the high interfacial tension between the two liquids.
The addition of chemicals to modify the properties of the flooding liquid is well known in the art. Surfactants are one class of chemical compounds that have been used in aqueous media for enhanced oil recovery. Surfactants have been found to effectively lower the interfacial tension between oil and water and enabling the oil droplets to flow more easily through the channels of the reservoir.
Alcohol ether sulfates are a class of anionic surfactants used in enhanced oil recovery. Alcohol ether sulfates are made by reacting alkoxylated alcohols with a sulfating agent to convert the alcohol functionality into a sulfuric acid semi-ester (—C—O—SO3H group). The sulfuric acid semi-esters are normally neutralized quickly after formation because they are generally not stable (see D. W. Roberts, Org. Process Res. Dev. 1998, 2, 194-202). As a result, alcohol ether sulfates tend to be difficult to manufacture.
It has now been found that preparation and neutralization of the sulfuric acid semi-esters is possible and in the presence of a co-solvent results in an alcohol ether sulfate product having improved neutralization and storage stability.